Host shifts of pathogens from animal species have lead to an increasing number of infectious diseases in humans. Studies of host shifts have been conducted primarily through the historical approach of tracing the epidemic's origin and identifying the reservoir host. While this is an important component of preventing disease transmission, the goals of the proposed research are to develop a more predictive approach to the study of host shifts and to attain a basic understanding of the ecological and genetic processes involved. The proposed research will combine phylogenetic data analysis with experimental microevolutionary studies to achieve these goals. The phylogenetic analyses will be used to determine the characteristics of hosts and pathogens that have favored host shifts in the past, and to use these characteristics to make predictions about the likelihood of future host shifts. Inoculation experiments will then be used to test these predictions. Studies will be carried out on the genetic and ecological basis of host shifts occurring in present day populations, and on the evolutionary and population dynamics of newly emerged pathogens. The fungal pathogen Microbotryum will be used as a model system. This pathogen is characterized by a large number of naturally occurring host-races, it is easily manipulated, and large experimental studies are feasible. This extensively studied natural plant pathogen system has already provided valuable insights into the biology of sexually transmitted diseases. This proposal will address these specific aims: 1. To use phylogenetic information to predict host shifts; and 2. To study host shifts in a population dynamic context, and thereby, a. Determine the population dynamics of pathogens on new hosts, b. Determine the role of genetic factors in the transmission of a pathogen to a new host, c. Determine the role of pathogen specialization in persistence on new hosts.